examples depending on configure output

The example direct_tcpip.c clearly cannot be compiled properly without
inclusion of a config.h or similar that is generated by configure so that the
HAVE_ defines exist properly.

But should we really "pollute" the examples by using something like that? It
will make them more complicated... But OTOH, a more complex example such as
that won't really be possible to do portable without using something
configure-like.